1. Field of the Invention
This invention relates to a method of and an apparatus for low-pressure casting, and more particularly to those which facilitate automation of the casting step.
2. Description of the Prior Art
Low-pressure casting has been wide used for mass production of, for instance, cylinder heads of automotive engines. In the low-pressure casting, molten light metal such as aluminum alloy held in a tightly enclosed vessel is cast in a cavity in a mold under a relatively low pressure of inert gas or air applied to the surface of the molten metal.
Generally the mold employed in such low-pressure casting is of metal and comprises a lower mold, an upper mold which is moved up and down toward and away from the lower mold and a pair of slide molds which are disposed between the upper and lower molds to be movable left and right and are slidably engaged with the upper and lower molds. When these molds are closed, a cavity having a shape conforming to the shape of a product to be cast is formed.
In such a casting mold, since the cavity is entirely surrounded by metal walls, molten metal cast in the mold is robbed of a large amount of heat upon contact with the cavity wall and especially the molten metal cast in a part of the cavity which defines a thin part of the product is very quickly cooled.
As a result, molten metal cannot be fed deep into the cavity or flow marks can be generated on the surface of the product. Further the mold can expand due to heat of the molten metal and the cavity can be partly narrowed, which can result in short molding and/or products with defects due to residual gas in the cavity.
When a sand mold having a small specific heat is used instead of the metal mold, temperature drop of the molten metal and short molding can be minimized. However since a pressure is applied to the molten metal during casting and solidifying step, the sand mold can be broken or moved. Accordingly, this approach cannot be applied to casting of precision products.
In order to overcome these problems, a novel method of low-pressure casting and a novel structure of a casting mold have been proposed, for instance, in Japanese Unexamined Patent Publication No. 63(1988)-72466.
In the method of low-pressure casting, the cavity is formed by a sand mold, and the sand mold is supported by metal molds. Then molten metal is cast in the cavity and is held under a pressure.
The structure of the casting mold comprises a sand mold forming a cavity, metal molds supporting the sand mold, and a gas discharge passage which communicates with the sand mold through the metal molds and discharges gas in the cavity.
However in the method and the structure of the casting mold, since the cavity is wholly formed by the sand mold, the cast product remains on the lower mold held in the sand mold after the casting mold is opened with the upper mold moved upward and the slide molds moved away from each other.
Therefore, a troublesome work and a long time are required to remove the cast product and the sand mold from the lower mold. Further the sand scattered over the upper surface of the lower mold when the sand mold is removed from the lower mold must be cleared off the lower mold, which takes a long time. These problems prevents shortening of the casting cycle and obstructs automation of the casting step.